1. Field of the Invention
The present invention relates to methods for the production of composite rigid material, and particularly to the forming of a rigid material from comminuted wood particles of various shapes.
2. Description of the Prior Art
The manufacture of composite rigid materials from comminuted particles, including wood chips or smaller particles, has been known for some time in the prior art. Particle board production has grown rapidly in the last 20 years and exceeded 45 million cubic meters in 1977. The main areas of application are the furniture and construction industries. Most particle board is made with urea-formaldehyde resin as a binder and is used for furniture manufacture and interior joinery work. Such material however has limited water resistance, and is not suitable for outdoor use. One further development came with the utilization of phenolic glues for the production of particle board, which lead to a greatly improved moisture-resistance.
In considering the prior art, it is important to note the types of materials which have been used in the production of these composite products. One such product is typically known in the art as particle board, and this is a board or other material made by gluing together relatively fine wood particles. Such particle board typically has a density of 42-45 pounds per cubic foot, is dried to a 6-8 percent moisture content, and has approximately 10 percent resin as the binder. Particle board is made on hot presses which act to eliminate a substantial amount of moisture, and also to set the binding glue. Another type of prior art product made from fine furnish material is medium density fibreboard, which includes lint-like particles.
Another primary type of prior art product is the type which typically comprises flake board or wafer board. About the coarsest size of the flakes would have a thickness from 0.005 inches to 0.015 inches. These flakes or wafers are carefully prepared to this range of thicknesses, and the resultant flake board is relatively dense as a result. This material includes products also referred to as wafer board, strand board, or oriented strand board. The fourth major category of prior art material is plywood.
As a means of comparison, the particle board and flake board are prepared in a comparable fashion in that a mat of loose particles and binder is compressed into the final board form. In the case of particle board, an initial matting of approximately four inches is compressed to a three-quarter inch thickness, a ratio of about five or six to one. Flake board is typically prepared by compressing a three inch mat of flakes to a thickness of three-quarters of an inch in the final board, a ratio of about four to one. Put in other terms, the particle board is compressed by about 80 percent, or in other words to a thickness of about 20 percent of its original thickness in the loose mat form, and the flake board is compressed by about 75 percent, or to a thickness of about 25 percent of the loose mat form. In contrast, the present invention uses irregularly shaped particles, having a diversity of thicknesses outside of the ranges of either of these prior art products, and the compression is preferably by at most about 65 percent, or to a volume preferably of not less than about 35 percent of the loose mat form.
Certain problems or disadvantages are associated with some of the prior art materials and methods. Of course, the prior art products and methods are well suited for particular applications and will continue to be so. However, these prior art processes can have substantial expenses associated with them which are avoided by the present invention. For example, the preparation of flake board involves the use of a flaker which is an expensive piece of equipment requiring a significant amount of maintenance and refurbishing from time to time. Additionally, the prior art methods have typically included equipment necessary to accurately screen and size the particles used in these prior art products, with additional equipment being necessary to handle the product not falling within the size specifications. Further, a greater amount of glue is typically required in the prior art because of the greater surface area needed to be bonded. Further, the density or compactness of these prior art products makes the moisture content more critical because it is less able to escape after production or to be handled during the process of production.
A variety of devices have been proposed and utilized in the prior art for the production of composite materials. In U.S. Pat. No. 3,565,725, issued to Siempelkamp of Feb. 23, 1971, there is disclosed a multi-level press for the production of such materials. According to the Siempelkamp patent, a fiber layer is deposited on a continuous surface and is then stacked and fed in several layers into the multi-level press. A related multi-platen press is disclosed in U.S. Pat. No. 3,050,777 issued to Siempelkamp on Aug. 28, 1962. A press for use in the preparation of pressed board is described in U.S. Pat. No. 3,892,619, issued to Hutz on July 1, 1975. In accordance with the Hutz description, a loosely coherent mass of comminuted material is formed and a lamina is added thereon. This combination is hot pressed between two surfaces. The ultimate product includes successive layers of fine, coarse and fine comminuted materials with the lamina being located on one of the fine material layers. In U.S. Pat. No. 3,482,675, issued to Hutz on Dec. 9, 1969, there is described a twin conveyor system for use in transporting mats of the described type.
Various resin materials have also been proposed in the prior art, and many of these are well understood as useful in the preparation of composite materials of this type. A molding powder including 20-30 weight percent of cellulosic filler, aliphatic guanamine- (or melamine-) formaldehyde reaction product acid accelerator and a lubricant is disclosed in U.S. Pat. No. 3,428,585, issued to Schuller on Feb. 18, 1969. In U.S. Pat. No. 3,895,984, issued to Cone et al. on July 22, 1975, there is described a method for continuously applying foamed glue to surfaces of a succession of wood veneers, layering the veneers, and then pressing them to form a plywood panel.